An X-ray image of the Sirius star
system located 8.6 light years from Earth. This
image shows two sources and a spike-like pattern due to
the support structure for the transmission grating. The
bright source is Sirius B, a white dwarf star that has
a surface temperature of about 25,000 degrees Celsius
which produces very low energy X-rays. The dim source
at the position of Sirius A – a normal star more
than twice as massive as the Sun – may be due to
ultraviolet radiation from Sirius A leaking through the
filter on the detector.

In contrast, Sirius A is the brightest star in the
northern sky when viewed with an optical telescope,
while Sirius B is 10,000 times dimmer. Because the two
stars are so close together Sirius B escaped detection
until 1862 when Alvan Clark discovered it while testing
one of the best optical telescopes in the world at that
time.

The theory of white dwarf stars was developed by S.
Chandrasekhar, the namesake of the Chandra X-ray
Observatory. The story of Sirius B came full cycle when
it was observed by Chandra in October 1999 during the
calibration or test period.

The white dwarf, Sirius B, has a mass equal to the
mass of the Sun, packed into a diameter that is 90%
that of the Earth. The gravity on the surface of Sirius
B is 400,000 times that of Earth!